The present invention relates to air ionizers and ionization systems and more particularly to air ionizers and ionization systems intended for use as electrostatic eliminators.
In certain applications, it is very desirable to reduce or eliminate electrostatic charge buildup on surfaces of objects, sometimes referred to as a target surfaces. Examples are surfaces in a "clean room" where precision manufacturing and assembly occurs such as electronic integrated circuit or optical equipment fabrication. In a typical clean room, the air flow in the room is closely controlled. A laminar air flow from ceiling to floor is utilized to sweep contaminant particles down and out of a room while limiting the side-to-side movement of particles. In addition, a reduction or elimination of electrostatic charge on critical target surfaces leads to cleaner surfaces, i.e., to surfaces which attract fewer particulates. Further, a reduction or elimination of electrostatic charge on critical target surfaces reduces the possibility of such charge directly damaging or destroying electrical integrated circuits by electrostatic discharge. Electrostatic charge buildups can be reduced or prevented by the presence of ionized air near the critical target surface. The charged air molecules combine with electrostatic charges existing on the target surface eliminating or reducing them.
One known mechanism for creating ionized air molecules is the use of a nuclear radiation source. Minnesota Mining and Manufacturing Company of Saint Paul, Minn., assignee of the present application, manufactures and markets a line of nuclear static eliminators, e.g., model 205 spot source or model 210 bar source, based upon the creation of air ions by the high energy alpha particles emitted by polonium 210 called radionuclides. The radionuclides are contained in a multiplicity of ceramic beads which in turn are held in an epoxy resin. U.S. Pat. No. 3,329,817, Walz, entitled, Radiation Source, describes an exemplary nuclear radiation source which patent is hereby incorporated by reference. Nuclear air ionizers, by their very nature, produce a balance, i.e., an equal number, of ions of both positive and negative polarity.
One problem with alpha radiation based nuclear ionization sources is that the nuclear ionization source must be located very close to, i.e., within about 1.5 inches (3.81 centimeters) of, the surface to which it is to affect. While nuclear ionization sources operate very satisfactorily for many applications, e.g. a particular spot along a conveyor in an assemblyline process, in other applications they do not work as satisfactorily. For example, in a clean room environment, it is desirable that a large area, or volume of air be ionized. This is very difficult to achieve with nuclear air ionizers due to their requirement of being very close to the target surface. Alternatively, air ions must be directed to the target surface by an air stream, i.e. a fan, which is not desirable due in part to the possibility of the air stream dislodging contaminates from surfaces.
Another known mechanism for creating ionized air molecules is the use of electric ionizers. Electric ionizers utilize a needle, i.e., a sharp point, to which high voltage is connected. Examples of electrical ionizers include those described in U.S. Pat. No. 4,306,271, Weber; U.S. Pat. No. 3,936,698, Meyer; U.S. Pat. No. 3,711,743, Bolasny; and Russian Pat. No. 842,347. Further, U.S. Pat. No. 4,542,434, Gehlke et al, entitled, Method and Apparatus for Sequenced Bipolar Air Ionization, describes an electrical air ionizer with two spaced electrodes, one of which is connected to a pulsed positive high voltage generator and the other to a pulsed negative high voltage generator.
Electrical air ionizers can produce large quantities of ions in a short period of time. However, experiments have shown that electrical ionizers tend to lead toward an unbalanced state, i.e., more ions of one polarity than another, which can actually cause electrostatic buildup instead of preventing it. This problem is described in an article by Blitshteyn, M. and Shelton, S., Microcontamination, Vol. 3, No. 3, pages 47-52, 76 (1985). Further, degradation of the needles, or electrodes, over a period of time also can lead to particulate contamination. This is described in an article by Liu, B., Pui, D., Kinstley, W., and Fisher, W., "Characterization of Electronic Ionizers for Clean Rooms" presented at The Annual Meeting of the Institute of Environmental Sciences, 1985.
U.S. Pat. No. 2,723,349, Rylsky, entitled, Apparatus For Ionizing an Air Stream, describes an apparatus which utilizes an air stream and provides for the generation of primarily only negative ions for therapeutic purposes (condition of general well-being). The system in Rylsky involves generation of ions by the use of radium (nuclear) emmissions which generates ions of both positive and negative polarity. A high direct current potential is applied to the ionizing structure to make it a negative potential, thus, neutralizing positive ions generated by the radium and leaving only "desirable" negative ions. Note that Rylsky does not show or suggest an alternating high voltage source but only the direct current source. In column 3 at lines 38-46, Rylsky does teach that a direct current voltage of the opposite, i.e., positive, potential could be utilized if it were desired to allow positive ions to exist in the air stream. While Rylsky does suggest the use of either applying a positive potential for a negative potential to the ionizing structure he does not show, teach or suggest both polarities alternately.
U.S. Pat. No. 2,785,312, Martin, Jr., entitled Ion Generator Using Radioactive Material, describes an apparatus which is quite similar to Rylsky but which utilizes a polonium nuclear ion source. In column 3 at lines 36-47, Martin, Jr. suggests the connection of an electric potential of either potential, but not both to the ionizing structure. Again, as in Rylsky, the object of Martin, Jr. is to generate ions of primarly only of one polarity.
An article by Secker, "A New Antistatic System With Long Range Throwing Power", Industrial Applications Society, IEE-IAS Annual Meeting, Toronto, Ontario (Oct. 1-5, 1978) compares the range of radioactive powered antistatic bars with conventional electrical antistatic bars. Secker contains no suggestion of combining radioactive sources with electrical fields or sources.
Other ionizers are described in U.S. Pat. No. 3,711,743 which describes a single ionizing electrode which is switched between positive and negative high voltage states. Also, U.S. Pat. No. 2,264,495 teaches the switching of a single ionizing electrode between positive and negative ion generation in response to sensors which detect variations in concentrations of both types of ions of incoming air in order to maintain a predetermined ratio of positive and negative ions.